1. Field of the Invention
The present invention relates to an optical fiber used in the field of optical communications and, in particular, to a single-mode optical fiber having a low dispersion slope.
2. Related Background Art
Conventionally, optical communication systems employing a single-mode optical fiber (hereinafter referred to as S-mode optical fiber) as their transmission line have often utilized light in the wavelength band of 1.3 .mu.m or 1.55 .mu.m as their signal light for communications. Recently, in order to reduce transmission loss in the transmission line, the light in the wavelength band of 1.55 .mu.m has been in use more and more. The S-mode optical fiber employed in such a transmission line for light in the wavelength band of 1.55 .mu.m (hereinafter referred to as 1.55-.mu.m S-mode optical fiber) has been designed such that its wavelength dispersion (phenomenon in which pulse wave spreads due to the fact that velocity of propagation of light changes depending on its wavelength) is nullified (namely, to yield a dispersion-shifted fiber whose zero-dispersion wavelength is 1.55 .mu.m). For example, as such a dispersion-shifted fiber, Japanese Patent Publication No. 3-18161 discloses a dispersion-shifted fiber having a dual-shape-core type refractive index profile. Also, a dispersion-shifted fiber having a segmented-core type refractive index profile is disclosed in "Relation between Macrobending Losses and Cutoff Wavelength in Dispersion-Shifted Segmented-Core Fiber," Electronics Letter, Vol. 22, No. 11, p. 574, 1986.
In such a conventional S-mode optical fiber having a dual-shape or segmented-core type refractive index profile, its optical core comprises an inner core occupying a center portion and having the highest average refractive index; and an outer core, disposed around the outer periphery of the inner core, having an average refractive index lower than that of the inner core but higher than that of its surrounding cladding. The inner and outer cores are in close contact with each other in the S-mode optical fiber having a dual-shape type refractive index profile, whereas they are separated from each other in the S-mode optical fiber having a segmented-core type refractive index profile.
Formed around the optical core constituted by the inner and outer cores is a cladding having a refractive index lower than the average refractive index of the outer core.
The cladding, which is a part constituting the outermost layer of the optical fiber, is often made of pure silica from the viewpoint of weather resistance and the like. The inner and outer cores, which ought to have an average refractive index higher than that of the cladding, are usually doped with GeO.sub.2 or the like in order to enhance the refractive index. Also, known as an alternative method of adjusting the refractive index is a method in which, for example, the cladding region is doped with fluorine such that the cladding region has a refractive index relatively lower than that of the core region.